Why would people put some drops of bat blood under a woman's pillow?

"In Roman antiquity, Pliny maintained that a man could stimulate a woman's desire by placing a clot of bat blood under her pillow."

http://www.batcon.org/batsmag/v10n3-5.html

It was believed that if you placed a few drops of bat's blood under a woman's pillow, she would bear many children (which could be a consequence of Leppi's answer!)

Good answers both.